Nuclear magnetic resonance (NMR) is a physical phenomenon involving quantum mechanical magnetic properties of atomic nuclei in the presence of an applied, external magnetic field. NMR phenomena can be observed with an NMR spectrometer and used to study molecular physics, crystalline and non-crystalline materials. In particular, nuclear spin phenomena can be used to generate a spectrum comprised of a pattern of lines representing the various spins and spin interactions.
In solid materials, the nuclear spins experience a great number of interactions that produce very broad and featureless lines. However, the interactions are time-dependent and can be averaged by physically spinning the sample (at high rotation speeds up to 80 kHz) at an inclination of the so-called magic angle) (54.74°) with respect to the direction of the external magnetic field. The averaging causes the normally broad lines become narrower, increasing the resolution for better identification and analysis of the spectrum.
To perform a magic angle spinning (MAS) nuclear magnetic resonance experiment, a sample is typically packed into a rotor that is, in turn, inserted into a stator assembly 100, such as that shown in FIG. 1. The cylindrical rotor 102 is axially symmetric about the axis of rotation 104 and typically fabricated from zirconia, silicon nitride or a polymer, such as poly-methyl-methacrylate or polyoxymethylene. The rotor 100 is sealed with a double or single end cap 106 fabricated from a material such as Kel-F, Vespel, zirconia or boron nitride depending on the application. The rotor 102 is held radially by a plurality of longitudinally spaced bearings 114 and 116 and is held longitudinally by a “Bernoulli” bearing located at one end. In this type of bearing a flat rotor end 108 engages a flat stator surface 110. A gas feed hole 112 through the flat stator surface 110 produces a gas flow in the circular space between the stator 110 and the rotor surface 108 which flow, in turn, produces a Bernoulli effect that results in a stable axial bearing position. The rotor is rotated by a compressed gas turbine system. In this system, a compressed gas introduced via orifice 118 expands and impinges on turbine blades 120 which are part of, or attached to, end cap 106.
The high rotational speed requires rotors to be radially and axially balanced before and during rotation and the rotation must occur without mechanical contact between stationary and rotating surfaces in order to reduce friction and prevent wear. Oil or liquids cannot be used in the spaces 122 and 124 between the rotor 102 and the radial bearings 114 and 116 because sufficient friction is created by the liquid to interfere with the rotation. Consequently, pressurized gas is normally introduced into these spaces in order to keep the outer walls of rotor 102 enveloped by a layer of gas before and during rotation.
The pressurized gas is introduced into the spaces between the rotor 102 and the bearings 114 and 116 by internal orifices in the bearings. These orifices are shown in more detail in FIGS. 2A and 2B which respectively show a front view of radial bearing 114 and a corresponding cross-sectional view taken along section line A-A of FIG. 2A. Bearing 114 has an annular groove 202 in its outer periphery 204 into which pressurized gas is introduced. The gas enters a plurality of inlet channels, for example, channels 208 and 210. In FIG. 2A, eight channels are shown symmetrically arranged around the central aperture 206 into which the rotor is inserted. Each inlet channel has, at its inner end, a nozzle that injects pressurized gas into the central aperture 206. For example, inlet channels 208 and 210 have nozzles 212 and 214, respectively. The eight nozzles provide a uniform gas layer with balanced pressure between the bearing and the rotor. Other designs may have more or less nozzles.
Orifice bearings of the type shown in FIGS. 2A and 2B fabricated from ceramic materials are conventionally used for NMR MAS stator assemblies. The ceramic material possesses an inherent hardness and stability over a wide temperature range that is generally adequate for satisfactory operation. However, due to the precise machining required of the hard ceramic material, the bearings are difficult and expensive to produce. In addition, during operation contaminates can block one or more of the nozzles. This situation creates pressure disturbances and disrupts the pressure balance. As a result, the rotor begins to wobble and is eventually destroyed as friction increases.